Friday, April 20, 2012

Gas Turbine in a Liquid Air Economy

   Thermodynamic cycle efficiency of an expansion engine is a function of the temperature difference between source and sink relative to the temperature of either. Just as heat source energy is stored in fuel, a phase change medium or the surrounding environment, so may heat sink energy be stored, particularly in a cryogenic phase change medium. The engine described here operates with combined source and sink storage, however source temperature may be as low as the ambient air environment. It should not be confused with a dual fuel engine. The gas turbine is selected to illustrate combined source and sink storage because of the simplification afforded by external compression.
   Refrigerant phase change storage using liquid air (nitrogen) is gaining acceptance as indicated by Highview Energy's operating 300 kW pilot plant at a Scottish and Southern power station outside London [1] and by a recent UK Integrated Delivery Programme grant for a "liquid air  engine to demonstrate disruptive technology for low carbon vehicles" [2]. In addition, Highview has recently licensed its liquid air energy storage (LAES) technology to General Electric Oil and Gas for use with GE peaking turbines. LAES has several advantages over battery storage and can work independently or as working fluid in conjunction with hydrogen storage. Advantages include:
* consistent efficient performance;
* very long service life with minimal disposal requirements;
* universal availability;
* no toxic or limited resources;
* small vehicle to generating station capacity;
* developed technology and;
* low weight and capital cost.
   Advanced liquefiers are driven by low cost station off-peak and various sources of renewable energy. 
   High rotor speed has always limited acceptance of the gas turbine in low capacity applications. This is especially true in motor vehicles because of variable speed operation. High density of cryogenic air solves this in the proposed Cryo-GT by lowering compression ratio while increasing operating efficiency at  constant speed and during turn-down. Injection of liquid air into regeneratively cooled intake air provides quasi-isothermal compression work with a reduction from 55 % of turbine output to 15 %, and the external compression Cryo-GT readily adapts to cryo-compression. Finally, the Cryo-GT is more cryogen and fuel efficient than the engines of the UK program with application to a wider range of vehicle size. In comparison, the vehicle engine of the UK program  loses efficiency because of unrecoverable latent heat from injection of intermediate heat transfer fluid, as in a Rankine cycle. 
   In automotive application the Cryo-GT operating efficiency is 4 times higher than for a normally aspirated engine. In stationary application operating efficiency is 2 times as for a large Advanced Turbine and 2.5 times as for a micro-turbine. Implications of Cryo-GT fuel consumption on emissions and fuel selection with respect to storage, safety, and cost are profound. It is especially advantageous with synthesis of hydrogen, which can be cogenerated using waste heat of air liquefaction, reducing the solar heating requirement by nearly 70 %. In addition, the technology has the unique advantage of competitive cost at inception, primarily due to the price of liquid air at less than 3 US cents/lb [3] and convertibility of mass produced turbo-chargers to turbines. Compared to fuel cell technology, a hydrogen fueled Cryo-GT is more efficient, does not require ultra-high purity, and can be implemented years ahead. Operating efficiency obviously does not take into account the renewable energy required for air liquefaction, nor does it account for fuel refining, distribution and exploration. A complete evaluation of the Cryo-GT and the Liquid Air Economy must include the impact of reduced fuel consumption on alternate fuel development and emissions, handling safety associated with both fuel and liquid air, and alternate energy storage.
   Production of liquid air or nitrogen requires about 40% of Cryo-GT power based on a liquefier Figure of Merit of 0.5 [4], a number already in practice, however there are strong mitigating factors; 
* liquefier waste heat will provide cogeneration of fuel and space heating; 
* recoverable intermittent sources including wind, solar, off-peak grid, vehicle recovery and  geothermal will provide liquefaction of refrigerant with uniform geographical distribution; 
* liquid nitrogen is already a bi-product of the liquefied oxygen industry;
* liquefier technology is advancing with the developing liquefied natural gas industry and;
* the liquid air is regeneratively pre-cooled in the engine compression system and readily re-liquified by the heat of fusion sink.

   A Liquid Nitrogen Economy was proposed in the early 1970's [6] shortly after a cryogenic engine was patented [7]. Both the engine and the proposed Economy were limited to ambient engine inlet gas temperature. Subsequently some Rankine cycle expansion engines with sub-ambient compression have been built and tested. These include a fired stationary gas turbine [8] and a fuel-less liquid nitrogen engine with an ambient heated quasi-isothermal expander [9] utilizing a frost free-heat exchanger [10] for sub-compact vehicles. More advanced concepts have been proposed to reduce refrigerant consumption, including a Brayton cycle for fuel-less operation with ambient heating and sub-ambient cooling [11]. The author of this blog has proposed a trans-ambient cycle with addition of over-ambient heating [12, 13].

   Two modes of operation are considered for both a stationary and an automotive gas turbine;
* Power, in which the gas turbine operates in a trans-ambient cycle with quasi-isothermal compression due to liquid air injection.
* Storage, in which renewable energy drives a liquefier to produce liquid air.

Power Mode
   Refrigerated compression increases the source to sink temperature difference by injection of refrigerant upstream of the compressor. Stored energy of refrigerant is added by lowering the sink temperature just as stored energy of fuel is added by raising the source temperature. Regeneration increases cycle efficiency and is included between the sink and ambient to cool intake air just as for heating intake air to the source. Quasi-isothermal compression increases working fluid density while lowering compression work. The preferred refrigerant is liquid air or because it is universally available. Carnot operating efficiency is over 90% with typical turbine inlet gas temperature of 1600 F and liquid air temperature of -300 F. Even in an unheated system Carnot operating efficiency exceeds 70%. High Carnot operating efficiency translates to high efficiency of the actual cycle, increasing inversely with respect to compression ratio [6], except as limited by recuperator or regenerator effectiveness. In low capacity engines, operating efficiency of the Cryo-GT is about 2.5 times as compared to a micro-turbine and about 4 times as for a gasoline engine. Liquid air replaces between 60% and 80% of the fuel in vehicle application and between 40% and 60% in stationary application, depending upon engine size and compression ratio.

Storage Mode
   Low cost station off-peak and renewable sources are available to drive liquefiers for stationary and vehicle use including building amplified wind and various natural sources. Renewables available for on-stream vehicle re-liquefaction include vehicle draft, deceleration, shock and solar. It is important to economize refrigerant consumption, especially in motor vehicle use, because of refrigerant weight limitation. Low fuel consumption of the Cryo-GT is advantageous to development of alternative fuels, and cogeneration of fuel with waste heat of refrigerant liquefaction provides further advantage.
   Comparison of Cryo-GT performance with other engines needs to include both liquid air and fuel preparation energy to determine total efficiency. Preparation of fuel includes refining, distribution and exploration, while preparation of liquid air involves only refining (liquefaction), since it does not have to be distributed or explored for. Refining of fuel such as coal or gasoline presently requires intense energy use, generally not amenable to renewable energy input such as wind and solar. Preparation of liquid air may utilize several advanced processes, including; compression/wet expansion [14], magnetic and thermo-acoustic, which are amenable to wind, solar and other renewable input. T
he brake driven heat of fusion reliquefier (sink) will increase refrigerant mileage in vehicle use many times over.

                                                           Cryo-GT Design Features
*  The enabling heat source for low capacity stationary and vehicle Cryo-GT application features exhaust gas recirculation (EGR) by an auxiliary jet-compressor/rotary regenerator.
* The enabling heat sink for stationary Cryo-GT application features liquefied refrigerant injection (LRI) into an electrically driven compressor.
* The enabling heat sink for fuel-less Cryo-GT vehicle application features a brake driven heat of fusion re-liquefier (BFL).
* The enabling heat sink for fired Cryo-GT vehicle application features liquefied refrigerant injection (LRI) into an electrically driven compressor..

* A refrigerant economizing heat sink for stationary Cryo-GT application features a renewable energy driven heat of fusion re-liquefier (RFL).
* A refrigerant economizing heat sink for fired Cryo-GT vehicle application features a brake driven heat of fusion re-liquefier (BFL).

                                                      Performance and Energy Cost
   It is useful to re-define some terms associated with renewable energy before proceeding with a tabulation of results involving the dual fluid (fuel and oxidizer/coolant) liquid air engine. The terms "tank-to-wheel” and “well-to-wheel” [5] used to describe vehicle efficiency including refining, distribution and exploration of fuel, are used herein as “operating” and “total”, respectively, for inclusion of stationary engines, non-fuel heat addition and liquefaction of air.
   Table 1 presents operating efficiency, total efficiency, mass ratio of liquid air to gasoline (Lqa/G), and energy cost for a compact car at 50 mph. The Cryo-GT, with EGR and LRI, develops 8 kW at a pressure ratio = 1.5 and turbine inlet gas temperature = 1620 F. An Otto cycle engine is included for comparison.
                                                               Table 1
                Op. eff.   Ttl. eff.   Lqa/G -----Lqa-----   ------G------
                   (%)          (%)          --     (lb/mi) 
 (c/mi)  (mpg)   (c/mi)    
Cryo-GT      51          19           43       2.5         8       103         4                           
Otto            18           14           --         --         --         35       11         
Specific energy costs are $4.00 US/gal of gasoline and $0.25 US/gal (3.3c/lb) of liquid air. 
   Table 2 presents operating efficiency, total efficiency and mass flow of make-up liquid air (Lqa) for a fuel-less Cryo-GT powered compact car at 50 mphThe Cryo-GT, with BFL, develops 8 kW at pressure ratio = 2 and turbine inlet gas temperature = 100 FCompression and expansion are quasi-isothermal. An Otto cycle engine is included for comparison.
                                                              Table 2
                Op. eff.    Ttl. eff.  -----Lqa-----     ------G------
                   (%)           (%)     (lb/mi)  (c/mi)    (mpg)   
Cryo-GT     65             21        0.2       0.7          --         --
Otto            18            14          --        --           35        14
Cost of installed 
liquid air is $0.25 US/gal (3.3c/lb)
   Table 3 presents operating fuel efficiency, total fuel efficiency and mass ratio of make-up liquid air to natural gas (Lqa/NG) of a low pressure Cryo-GT for distributed generation. The Cryo-GT, with EGR and RFL, develops 30 kW at a pressure ratio = 3 and turbine inlet gas temperature = 1620 F. A micro-turbine is included for comparison.                                                  
                                                             Table 3
                     Op.Eff.  Ttl.Eff.  Lqa/NG  ------Lqa------  ------NG------
                       (%)        (%)           --       (lb/hr) (c/kWhr)  (lb/hr) (c/kWhr)  
LP Cryo-GT    74         43            3.3         23        2.5         6.9       2.3
Micro-GT       28         21             --          --         --           18        6.1
Specific energy costs are $0.50 US/therm NG and $0.25 US/gal (3.3c/lb) of liquid air. 

   Table 4 presents operating fuel efficiency, total fuel efficiency and mass ratio of make-up liquid air to natural gas (Lqa/NG) of a high pressure stationary Cryo-GT. The Cryo-GT, with RFL, develops 200 MW at a pressure ratio = 18 and turbine inlet gas temperature = 2900 FAn Advanced Turbine is included for comparison.                                                  
                                                             Table 4
                     Op.Eff.  Ttl.Eff.  Lqa/NG  ------Lqa------  ------NG------
                       (%)        (%)          --       (lb/hr) (c/kWhr)  (lb/hr) (c/kWhr)  
HP Cryo-GT   82          25                                                 41K       2.1
Adv.-GT         40          29           --          --          --         84K       4.3
Specific energy costs are $0.50 US/therm NG and $0.25 US/gal (3.3c/lb) of liquid air. 

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